A blue whale's song can travel huge distances. This means he or she can communicate with another whale swimming up to hundreds of miles away.

Scientists have noticed that the frequency of whale calls are lower today than they were just twenty years ago, and they have no idea why. They've also recorded baleen whale sounds in the North Atlantic missing the "overtone" portion of the call. Again, scientists are dumbfounded.

New Research published in Science Reports, however, suggests that the whales are choosing to do this – and that human activity is probably to blame.

Before now, it was believed a whale's call is generated by a resonating sound in the chambers of the animal's upper respiratory system. If this was the case, the frequency would depend entirely on the size of the whale – the bigger the whale, the lower the frequency of its call. But researchers at Oregan State University's Hatfield Marine Center have proved this theory wrong.

First, they recorded a blue whale's call, then they built a model to try and mimic the sound.

"We tried to envision a mechanism whereby whales could gradually lower the frequency of their calls through time, or produce calls with unusual harmonic structure, by only resonating sound in their upper respiratory chamber – and it was physically impossible," said Robert Dziak, lead author and acoustics scientist with the National Oceanic and Atmospheric Administration (NOAA), in a statement.

"Only when we pulsed air through the process of opening and closing the vocal cords did we get a way to produce sounds that can change frequencies in mid-call as well as remove overtones."

So whales can mix up the frequency of their calls by changing the rate they blow air through their vocal cords. 

"That also suggests that the change in the frequency might be cognitive. They are choosing to make it higher or lower in response to some sort of environmental stimulus," added Dziak.

But why the frequency change? One suggestion is that it's a response to an increase in human-generated sound. Whales are changing the frequency of their call to a radio channel with less static, if you will.

Or it could be because there are more blue whales than there were a couple of decades ago. Don't get too excited though, they're still listed on the IUCN Red List of endangered animals and new research also shows thirty years after the hunting ban, they're not recovering as fast as thought.

"Lower-frequency sounds can be produced at lower intensity by the animal than high-frequency sounds and yet low-frequency sound still travels further," said Dziak.

"Those factors may also play a role in the vocalization changes over the past two decades."